Chapter5-6_Fall2011_IK

Chapter5-6_Fall2011_IK - Chapter 5: The Data Link Layer Our...

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Chapter 5: The Data Link Layer Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies
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ink Layer: Introduction ome terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs layer-2 packet is a frame , encapsulates datagram “link” ata-link layer has responsibility of ransferring datagram from one node o adjacent node over a link
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Link layer: context Datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link Each link protocol provides different services e.g., may or may not provide rdt over link transportation analogy trip from Princeton to Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm
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Link Layer Services Framing, link access: encapsulate datagram into frame, adding header, trailer channel access if shared medium “MAC” addresses used in frame headers to identify source, dest • different from IP address! Reliable delivery between adjacent nodes we learned how to do this already (chapter 3)! seldom used on low bit error link (fiber, some twisted pair) wireless links: high error rates • Q: why both link-level and end-end reliability?
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Link Layer Services (more) Flow Control: pacing between adjacent sending and receiving nodes Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors: • signals sender for retransmission or drops frame Error Correction: receiver identifies and corrects bit error(s) without resorting to retransmission Half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but not at same time
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Adapters Communicating link layer implemented in “adapter” (aka NIC) Ethernet card, PCMCIA card, 802.11 card sending side: encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to rcving node adapter is semi- autonomous implements link & physical layers ending ode frame rcving node datagram frame adapter adapter link layer protocol
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Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable!
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This note was uploaded on 01/09/2012 for the course CS cs464 at Bilkent University.

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Chapter5-6_Fall2011_IK - Chapter 5: The Data Link Layer Our...

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